1. Field of the Invention
The present invention relates to an electret condenser microphone, and specifically, a compact electret condenser microphone having excellent frequency characteristics and high sensitivity.
2. Related Background Art
A condenser microphone mainly comprises a diaphragm that vibrates in response to sound waves and a fixed electrode that faces the diaphragm with a slight gap therebetween. The diaphragm and the fixed electrode define a capacitor. When the diaphragm vibrates in response to the sound waves, the capacitance of the capacitor varies. For example, the condenser microphone outputs a variation in capacitance of the condenser microphone as a variation in voltage. The condenser microphone has any directivity by modifying its structure.
The diaphragm of the condenser microphone is composed of a metalized synthetic-resin thin film. A back electret microphone has an electret dielectric film on the surface (facing the diaphragm) of the fixed electrode. A film electrets microphone has an electret dielectric film on the surface of a diaphragm.
FIGS. 4 to 6 illustrate an example conventional condenser microphone which is substantially the same as that disclosed in Japanese Patent No. 4110068. In FIGS. 4 to 6, the condenser microphone includes a microphone capsule 10 and a microphone case 20 which are detachable from each other by turning a male-thread joint ring 30. The microphone capsule 10 has a cylindrical capsule case 11 composed of metal such as brass. A front acoustic terminal 111 is provided on the front end surface of the capsule case 11 while a rear acoustic terminal 112 is adjacent the rear end of peripheral surface of the capsule case 11. The front acoustic terminal 111 has slit openings. The rear acoustic terminal 112 includes a plurality of circular holes disposed at regular intervals in a circumferential direction.
The microphone case 20 is a cylindrical body made of metal such as brass and houses a circuit board 21 therein which includes an FET 211 serving as an impedance converter. A microphone cable 23 is fixed to the rear end of the microphone case 20 optionally with a clamp or a cable bushing. A female thread 114 that is screwed into the male-thread joint ring 30 is provided on the inner peripheral surface adjacent a rear opening of the capsule case 11. A female thread 204 that is screwed into the male-thread joint ring 30 is provided on the inner peripheral surface adjacent a front opening of the microphone case 20.
A diaphragm 12, a spacer 122, a fixed electrode 13, a support 14 that supports the fixed electrode 13 and an insulating sleeve 16 are inserted into the capsule case 11 in this order from the rear opening of the capsule case 11. The capsule case 11 is provided with a side mesh 17 which protects the rear acoustic terminal 112 against intrusion of dust and foreign substances but does not function as an acoustic resistor. The diaphragm 12 is composed of a metalized synthetic-resin thin film. The diaphragm 12 is fixed to a support ring 121 made of brass or the like under predetermined tension and is housed in the capsule case 11 via the support ring 121. A step 113 in the capsule case 11 positions the support ring 121.
A fixed electrode 13 of an electret board composed of an aluminum plate on which an electret material of fluorinated ethylene propylene resin (FEP) or the like having a thickness of about 25 μm has self polarization effects and requires no polarization power supply. A fixed electrode 13 composed of a metallic plate, however, needs polarization power supply. The fixed electrode 13 has a predetermined number of holes extending from the front surface to the back surface of the electrode. The diaphragm 12 is composed of a metalized synthetic-resin thin film. The fixed electrode 13 of a back electret microphone has an electret dielectric film on the surface (facing the diaphragm) thereof. The diaphragm 12 of a film electrets microphone has an electret dielectric film on the surface thereof.
The support 14 includes a column having a large-diameter segment 141 that can support a fringe of the fixed electrode 13 and a small-diameter segment 142 concentrically integrated to the rear end of the support 14. An acoustic resistor 151 and a damper 152 are housed in the large-diameter segment 141. The acoustic resistor 151 is composed of a fine mesh material, for example. The damper 152 biases the acoustic resistor 151 against the bottom of the large-diameter segment 141 and is composed of air-permeable sponge, for example.
The small-diameter segment 142 has an enough length to contact to the gate of the FET 211 when the microphone capsule 10 and the microphone case 20 are connected. The inner peripheral surfaces of the large-diameter segment 141 and the small-diameter segment 142 communicate with each other. The small-diameter segment 142 has a plurality of sound inlets 143 sound from the rear acoustic terminal 112 enters. Accordingly, the sound passes from the rear acoustic terminal 112 to the small-diameter segment 142 through the sound inlets 143, and then the sound is introduced to the large-diameter segment 141, the fixed electrode 13 and then the back surface of the diaphragm 12 via through holes (not shown). On the way to the diaphragm 12, the sound is acoustically resisted by the acoustic resistor 151.
Sound leakage, which is the sound flow that reaches the back surface of the diaphragm 12 with the sound avoiding the acoustic resistor 151, is mainly generated by the sound passing from a step 144 (see FIG. 6) on the inner peripheral surface of the large-diameter segment 141 and the small-diameter segment 142 of the support 14 to the radial direction of the large diameter-columns 141. According to the example of the unidirectional microphone mentioned above, since the large-diameter and small-diameter segments 141, 142 are concentrically disposed in the longitudinal direction, the contact area between the step 144 and the acoustic resistor 151 can be increased by increasing the area of the step 144 even in a microphone capsule 10 having small diameter. Accordingly, the inner diameter of the large-diameter segment 141 is designed to be as large as possible, while the inner diameter of the small-diameter segment 142 is designed to be as small as possible. Accordingly, the fluctuation of the acoustic resistance against the sound from the rear acoustic terminal 112 to the diaphragm 12 is reduced and the small-diameter directional capacitor microphone has a small fluctuation in the directional frequency response, the sensitivity and the signal-to-noise ratio.
A unidirectional condenser microphone includes an acoustic cavity (air chamber) and an acoustic resistor on the back of the fixed electrode 13 for obtaining unidirectionality. Typically, the acoustic cavity (air chamber) is formed of an insulating spacer that supports the fixed electrode 13. The acoustic resistor is disposed on the rear opening of the insulating spacer so as to form an acoustic circuit for obtaining the unidirectionality. The unidirectional condenser microphone further includes an extraction electrode for leading signals from the fixed electrode 13. Conventionally, the support 14 having the large- and small-diameter segments 141, 142 is composed of a conducting material and serves as the extraction electrode for electrically connecting the fixed electrode 13 to the FET 211. The small-diameter segment 142 has a hole on its peripheral wall which serves as a rear acoustic terminal 112. The support 14 serves as an acoustic cavity (air chamber). Accordingly, a unidirectional condenser microphone having a small-diameter column can be achieved. These features of such a condenser microphone are used for an electret condenser microphone which has is self-polarization effects and needs no other polarization power supply.
The electret condenser microphone explained above includes a stray capacitor formed between the outer periphery of the diaphragm 12 which does not vibrate in response to sound and the fixed electrode 13, in addition to the capacitance of the capacitor formed between the diaphragm 12 and the fixed electrode 13. A large stray capacitance disadvantageously has affects of a reduction in sensitivity and an increase in distortion on the acoustic performances of the electret condenser microphone. A smaller condenser microphone is more significantly affected by the stray capacitance. Furthermore, in the electret condenser microphone disclosed in Japanese Patent No. 4110068, the support 14 biases the fixed electrode 13. If the support 14 biases the fixed electrode 13 with unexpected large biasing force upon assembling the microphone, the capacitance of the capacitor varies. Accordingly, the electret condenser microphone has disadvantages in decreasing the acoustic performances such as frequency characteristics or sensitivity.